WO2025095010A1 - Plated steel sheet - Google Patents

Abstract

Provided is a plated steel sheet characterized by comprising a base-material steel sheet, and a plating layer formed on a surface of the base-material steel sheet, wherein: the plating layer has a prescribed chemical composition; in a cross section of the plating layer, the length L of the interface between the plating layer and the base-material steel sheet and the length L₀ of the surface of the base-material steel sheet satisfy (L-L₀)/L₀×100 ≥ 3; the plating layer includes an Fe-Al phase; and the thickness of the Fe-Al phase is 4-50 μm.

Description

Galvanized Steel Sheet

The present invention relates to plated steel sheets.

Zn-based plated steel sheets are known to exhibit sacrificial corrosion protection and have excellent corrosion resistance. On the other hand, many proposals have been made for plated steel sheets with plating layers containing other elements instead of or in addition to Zn.

For example, Patent Document 1 describes an Al-based plated steel sheet comprising: a base steel sheet; a first alloy plating layer having a thickness of 3 to 30 μm and having a component composition containing, in mass%, 40 to 70% Fe, 0.3 to 10% Mn, and the balance being Al and unavoidable impurities, on at least one side of the base steel sheet; a second alloy plating layer having a thickness of 0.10 to 10 μm and having a component composition containing, in mass%, 5 to 50% Fe, 5 to 40% Mn, and the balance being Al and unavoidable impurities, on the first alloy plating layer; and unalloyed Al with a deposition amount of 0 to 1000 mg/ ^m2 deposited on the surface of the second alloy plating layer. Furthermore, Patent Document 1 teaches that by (i) subjecting a base steel sheet to hot-dip plating of an Al-Mn alloy to form two Al-Fe-Mn alloy plating layers having different Mn contents on the surface of the base steel sheet, and (ii) limiting the amount of unalloyed Al attached to the surface of the Al-Fe-Mn alloy plating layer to a range of 1000 mg/ ^m2 or less, it is possible to achieve both post-painting corrosion resistance and resistance spot weldability in an environment conforming to the corrosive environment of an automobile exterior panel.

JP 2020-122205 A

In the case of Al-based plated steel sheets as described in Patent Document 1, alloying treatment is generally required to ensure corrosion resistance after painting. However, because alloyed Al-based plating is relatively hard, the cold workability of the plated steel sheet may decrease.

The present invention aims to provide a plated steel sheet having an Al-containing plating layer, which has improved corrosion resistance and cold workability after painting.

As a result of investigations conducted by the inventors to achieve the above object, they discovered that it is possible to achieve improved corrosion resistance and cold workability after painting by optimizing the chemical composition of the Al-containing plating layer and by appropriately controlling the thickness of the Fe-Al phase contained in the Al-containing plating layer and the morphology of the interface between the Al-containing plating layer and the base steel sheet, and thus completed the present invention.

The present invention, which has achieved the above object, is as follows.
(1) A steel plate having a base steel sheet and a plating layer formed on a surface of the base steel sheet,
The plating layer comprises, in mass %,
Fe: 20.0 to 55.0%,
Mg: 0-10.0%,
Si: 0 to 10.0%,
Zn: 0-30.0%
and further comprising
Ni: 0-1.000%,
Ca: 0-4.000%,
Sb: 0 to 0.500%,
Pb: 0 to 0.500%,
Cu: 0 to 1.000%,
Sn: 0-1.000%,
Ti: 0 to 1.000%,
Cr: 0-1.000%,
Nb: 0 to 1.000%,
Zr: 0 to 1.000%,
Mn: 0 to 1.000%,
Mo: 0-1.000%,
Ag: 0-1.000%,
Li: 0 to 1.000%,
La: 0 to 0.500%,
Ce: 0-0.500%,
B: 0 to 0.500%,
Y: 0 to 0.500%,
Sr: 0-0.500%,
In: 0 to 0.500%,
Co: 0 to 0.500%,
Bi: 0-0.500%,
P: 0 to 0.500%,
W: 0 to 0.500%, and V: 0 to 0.500%
At least one of the above is contained in a total amount of 5.000% or less,
The balance has a chemical composition consisting of 20.0% or more Al and impurities,
In a cross section of the coating layer, an interface length L between the coating layer and the base steel sheet and a length L ₀ of a surface of the base steel sheet satisfy (L - L ₀ ) / L ₀ × 100 ≧ 3,
The plated steel sheet, characterized in that the plated layer contains an Fe-Al phase, and the thickness of the Fe-Al phase is 4 to 50 μm.
(2) The plated steel sheet according to the above (1), characterized in that (LL ₀ )/L ₀ ×100≧5.
(3) The plated steel sheet according to (2) above, characterized in that (LL ₀ )/L ₀ ×100≧7.
(4) The plated steel sheet according to any one of (1) to (3) above, characterized in that the Mg content in the plated layer is 0.2% or more.
(5) The chemical composition is, in mass%,
The plated steel sheet according to any one of (1) to (4) above, characterized in that it contains Mg: 0.3 to 10.0% and Si: 0 to 1.0%.
(6) The plated steel sheet according to any one of (1) to (5) above, characterized in that the thickness of the Fe—Al phase is 12 to 50 μm.
(7) The plated steel sheet according to any one of (1) to (6), characterized in that, in a cross section of the plated layer, a projected length T _i of the Fe—Al—Si phase in the plated layer and a length L ₀ of the surface of the base steel sheet satisfy ΣT _i /L ₀ × 100≦20.
(8) The plated steel sheet according to (7) above, wherein ΣT _i /L ₀ ×100≦1.
(9) The chemical composition contains, in mass%, Mg: 0.3 to 10.0%,
The plating layer further includes an Mg-containing phase,
The plated steel sheet according to any one of (1) to (8) above, characterized in that in a cross section of the plated layer, a surface coverage rate of the Mg-containing phase is 20 to 100%.
(10) The plated steel sheet according to (9) above, characterized in that the surface coverage of the Mg-containing phase is 60 to 100%.
(11) The plated steel sheet according to any one of (1) to (10) above, characterized in that the Mg content in the plated layer is 2.4% or less.
(12) The plated steel sheet according to any one of (1) to (11) above, characterized in that the Si content in the plated layer is 0.2% or more.
(13) The plated steel sheet according to any one of (1) to (12) above, characterized in that an area ratio of the MgZn ₂ phase in the plated layer is less than 10%.

According to the present invention, it is possible to provide a plated steel sheet having an Al-containing plating layer, which has improved corrosion resistance and cold workability after painting.

1 is a schematic cross-sectional view of a plated steel sheet according to an embodiment of the present invention, showing the interface length L between the plating layer and the base steel sheet, and the length _L0 of the surface of the base steel sheet. 1 is a schematic cross-sectional view of a plated steel sheet according to a preferred embodiment of the present invention, showing the projected length T _i of the Fe—Al—Si phase and the length L ₀ of the surface of the base steel sheet. FIG. 2 is a cross-sectional schematic view of a plated steel sheet according to another preferred embodiment of the present invention, illustrating the surface coverage of an Mg-containing phase.

<Plated steel sheet>
A plated steel sheet according to an embodiment of the present invention comprises a base steel sheet and a plating layer formed on a surface of the base steel sheet,
The plating layer comprises, in mass %,
Fe: 20.0 to 55.0%,
Mg: 0-10.0%,
Si: 0 to 10.0%,
Zn: 0-30.0%,
and further comprising
Ni: 0-1.000%,
Ca: 0-4.000%,
Sb: 0 to 0.500%,
Pb: 0 to 0.500%,
Cu: 0 to 1.000%,
Sn: 0-1.000%,
Ti: 0 to 1.000%,
Cr: 0-1.000%,
Nb: 0 to 1.000%,
Zr: 0 to 1.000%,
Mn: 0 to 1.000%,
Mo: 0-1.000%,
Ag: 0-1.000%,
Li: 0 to 1.000%,
La: 0 to 0.500%,
Ce: 0-0.500%,
B: 0 to 0.500%,
Y: 0 to 0.500%,
Sr: 0-0.500%,
In: 0 to 0.500%,
Co: 0 to 0.500%,
Bi: 0-0.500%,
P: 0 to 0.500%,
W: 0 to 0.500%, and V: 0 to 0.500%
At least one of the above is contained in a total amount of 5.000% or less,
The balance has a chemical composition consisting of 20.0% or more Al and impurities,
In a cross section of the coating layer, an interface length L between the coating layer and the base steel sheet and a length L ₀ of a surface of the base steel sheet satisfy (L - L ₀ ) / L ₀ × 100 ≧ 3,
The plating layer contains an Fe-Al phase, and the thickness of the Fe-Al phase is 4 to 50 μm.

As mentioned above, in order to ensure corrosion resistance after painting, it is generally necessary to perform an alloying treatment on Al-based plated steel sheets. However, because alloyed Al-based plating is relatively hard, the cold workability of the plated steel sheet may be reduced. For example, the alloyed Al-based plating may peel off into powder form during cold working (also known as powdering), i.e., powdering resistance may be reduced. Therefore, in plated steel sheets having a plating layer made of Al-based plating, it is generally difficult to achieve both corrosion resistance after painting and cold workability, especially powdering resistance.

The inventors therefore conducted research to achieve both post-painting corrosion resistance and cold workability in plated steel sheets having a plating layer made of Al-based plating, focusing in particular on the chemical composition, structure, and morphology of the plating layer. As a result, the inventors discovered that by optimizing the chemical composition of the plating layer and appropriately controlling the thickness of the Fe-Al phase contained in the plating layer and the morphology of the interface between the plating layer and the base steel sheet, it is possible to significantly improve both post-painting corrosion resistance and cold workability.

More specifically, the inventors first discovered that by setting the Fe content in the plating layer to 20.0 mass% or more and controlling the thickness of the Fe-Al phase contained in the plating layer to 4 μm or more, the plating layer can be sufficiently alloyed, thereby improving the post-painting corrosion resistance of the plated steel sheet. On the other hand, the inventors discovered that by controlling the thickness of the Fe-Al phase to 50 μm or less and suppressing excessive hardening of the plating layer, the cold workability of the plated steel sheet can be improved.

Next, the present inventors have studied the form of the plating layer in order to further improve the cold workability of the plated steel sheet. As a result, the present inventors have found that the cold workability of the plated steel sheet can be significantly improved by controlling the interface shape between the plating layer and the base steel sheet to a shape with greater irregularities, more specifically, by controlling the interface length L between the plating layer and the base steel sheet and the length _{L 0} of the surface of the base steel sheet to a shape with greater irregularities that satisfies the relationship (L - L ₀ ) / L 0 × 100 ≧ 3. FIG. 1 is a cross-sectional schematic diagram of a plated steel sheet according to an embodiment of the present invention, showing the interface length L between the plating layer and the base steel sheet and the length L ₀ of the surface of the base steel sheet. Referring to FIG. 1, a plated steel sheet 1 according to an embodiment of the present invention includes a base steel sheet 2 and a plating layer 3 formed on the surface of the base steel sheet 2, and the plating layer 3 includes an Fe-Al phase 4. 1 , the interface length L between the coating layer 3 and the base steel sheet 2 and the corresponding length _L0 of the surface of the base steel sheet 2 satisfy the relationship of ( _LL0 )/ _LL0 × 100≧3, that is, the interface length L is 3% or more longer than the length _L0 of the surface of the base steel sheet 2. It can therefore be seen that the interface between the coating layer 3 and the base steel sheet 2 is controlled to have a shape with greater irregularities.

Without intending to be bound by any particular theory, it is believed that by having the interface between the plating layer 3 and the base steel sheet 2 have a shape with greater irregularities as shown in FIG. 1, the hard plating layer 3 can bite into the base steel sheet 2 starting from the irregularities at the interface during cold working such as bending, and can proceed with cold working while deforming the base steel sheet 2. As a result, it is possible to significantly suppress the occurrence of powdering due to bending, etc., that is, it is possible to significantly improve the cold workability of the plating steel sheet 1. On the other hand, if the interface between the plating layer 3 and the base steel sheet 2 has a flat shape or a flatter shape with less irregularities, it is not possible to proceed with cold working such as bending by having the hard plating layer 3 bite into the base steel sheet 2, and therefore it is not possible to sufficiently suppress the occurrence of powdering.

The present inventors have also found that in order to create an interface shape with greater irregularities between the plating layer 3 and the base steel sheet 2, it is effective to increase the alloying rate during the alloying treatment of the plating layer 3. To explain in more detail, first, if the plating layer 3 contains excessive amounts of Si and Mg, it may adversely affect the alloying of the plating layer 3, so that the Si and Mg contents in the plating layer 3 must be controlled to 10.0 mass% or less, respectively. In addition, in order to increase the alloying rate of the plating layer 3, it is necessary to appropriately control the metal structure of the base steel sheet 2 during the alloying treatment. More specifically, by making the base steel sheet 2 during the alloying treatment into a metal structure that is moderately decarburized and contains a larger amount of austenite phase, the reaction between the plating layer 3 and the austenite phase in the base steel sheet 2 during the alloying treatment is promoted, that is, it is possible to significantly increase the alloying rate. As will be described in detail later in relation to the manufacturing method of the plated steel sheet 1, the inventors have found that it is possible to create a metal structure of the base steel sheet 2 that is moderately decarburized and contains a larger amount of austenite phase by appropriately controlling the annealing step, cooling step, and plating step of the base steel sheet 2. As a result, it is possible to realize an interface shape with greater irregularities such that the interface length L between the plating layer 3 and the base steel sheet 2 and the corresponding length L ₀ of the surface of the base steel sheet 2 satisfy the relationship (L - L ₀ ) / L ₀ × 100 ≧ 3, thereby making it possible to significantly improve the cold workability of the plated steel sheet 1.

In particular, the inventors have now revealed for the first time that controlling the Fe-Al phase 4 in an appropriately alloyed coating layer 3 to within a range of 4 to 50 μm ensures sufficient corrosion resistance after painting and improves cold workability, and further controlling the interface shape between the coating layer 3 and the base steel sheet 2 to a shape with greater irregularities that satisfies the relationship (LL ₀ )/L ₀ × 100≧3 can significantly improve the cold workability of the coated steel sheet 1. Therefore, the coated steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where both corrosion resistance after painting and cold workability are required.

Below, the plated steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values before and after it are included as the lower and upper limits, unless otherwise specified.

[Plating layer]
According to an embodiment of the present invention, the plating layer is formed on the surface of the base steel sheet, for example, on at least one surface, preferably both surfaces, of the base steel sheet. The plating layer has the following chemical composition.

[Fe:20.0-55.0%]
When a plated steel sheet is alloyed, Fe from the base steel sheet diffuses into the plated layer and is alloyed with Al, etc., so that the plated layer inevitably contains Fe. In order to ensure corrosion resistance after painting, it is necessary to appropriately alloy the plated steel sheet, and therefore the Fe content is set to 20.0% or more. The Fe content may be 25.0% or more, 30.0% or more, 35.0% or more, or 40.0% or more. On the other hand, if the Fe content is too high, the cold workability may be reduced due to excessive alloying of the plated layer. Therefore, the Fe content is set to 55.0% or less. The Fe content may be 52.0% or less, 50.0% or less, 48.0% or less, or 45.0% or less.

[Mg: 0-10.0%]
Mg is an element effective in improving the corrosion resistance of the plating layer, particularly the chemical conversion treatability. The Mg content may be 0%, but in order to obtain such an effect, the Mg content is preferably 0.2% or more. The Mg content may be 0.3% or more, 0.5% or more, 0.8% or more, 1.0% or more, 1.5% or more, or 2.0% or more. On the other hand, if Mg is contained excessively, the alloying speed during alloying treatment of the plating layer may be slowed down, and in this case, the desired interface shape cannot be obtained between the plating layer and the base steel sheet. Therefore, the Mg content is set to 10.0% or less. The Mg content may be 8.0% or less, 6.0% or less, 5.0% or less, 4.0% or less, 3.0% or less, less than 2.5%, 2.4% or less, or 2.2% or less.

[Si: 0-10.0%]
Si is an element effective in improving the adhesion of the plating layer. The Si content may be 0%, but in order to fully obtain such an effect, the Si content is preferably 0.1% or more. The Si content may be 0.2% or more, 0.3% or more, 0.5% or more, 0.6% or more, or 0.8% or more. On the other hand, if Si is contained excessively, the alloying speed during alloying treatment of the plating layer may be slowed down, and in this case, the desired interface shape cannot be obtained between the plating layer and the base steel sheet. Therefore, the Si content is set to 10.0% or less. The Si content may be 8.0% or less, 6.0% or less, 4.0% or less, or 2.0% or less. By further reducing the Si content, it is possible to significantly suppress or reduce the formation of Fe-Al-based intermetallic compounds containing a relatively large amount of Si, more specifically, Fe-Al-Si phases containing 3 mass% or more of Si. If the Fe-Al-Si phase is present in a relatively large amount, bimetallic corrosion (galvanic corrosion) may occur between the Fe-Al phase (content of elements other than Fe, Al and Zn is less than 3%). Therefore, from the viewpoint of further improving corrosion resistance, the Si content is preferably 1.0% or less.

[Zn: 0-30.0%]
Zn has a sacrificial anticorrosive effect and is an element effective in improving the corrosion resistance of the plating layer. The Zn content may be 0%, but in order to fully obtain such an effect, the Zn content is preferably 1.0% or more. The Zn content may be 3.0% or more, 5.0% or more, 10.0% or more, 12.0% or more, 15.0% or more, or 18.0% or more. On the other hand, if Zn is contained excessively, the melting of Zn during welding of the plated steel sheet becomes significant, and the molten Zn may penetrate into the steel and cause liquid metal embrittlement (LME) cracking. Therefore, the Zn content is preferably 30.0% or less. The Zn content may be 28.0% or less, 25.0% or less, 22.0% or less, or 20.0% or less.

Furthermore, the plating layer may optionally contain Ni: 0-1.000%, Ca: 0-4.000%, Sb: 0-0.500%, Pb: 0-0.500%, Cu: 0-1.000%, Sn: 0-1.000%, Ti: 0-1.000%, Cr: 0-1.000%, Nb: 0-1.000%, Zr: 0-1.000%, Mn: 0-1.000%, Mo: 0-1.000%, Ag: 0 At least one of the following may be contained: 0 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0 to 0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, W: 0 to 0.500%, and V: 0 to 0.500%. These optional elements are not particularly limited, but are preferably 5.000% or less in total. The optional elements may be a total of 4.500% or less, 4.000% or less, 3.500% or less, 3.000% or less, 2.500% or less, 2.000% or less, 1.500% or less, 1.000% or less, 0.800% or less, 0.500% or less, 0.100% or less, or 0.050% or less. These optional elements are described in detail below.

[Ni: 0-1.000%]
Ni is an element effective for improving the corrosion resistance of the plating layer. The Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.003% or more, 0.005% or more, 0.008% or more, 0.010% or more, or 0.020% or more. Although there is no particular upper limit, from the viewpoint of manufacturing costs, etc., the Ni content is set to 1.000% or less, and may be, for example, 0.500% or less, 0.400% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[Ca: 0-4.000%]
Ca is an element effective in ensuring wettability of the plating bath. The Ca content may be 0%, but in order to obtain such an effect, the Ca content is preferably 0.001% or more. The Ca content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, if Ca is contained excessively, a large amount of hard intermetallic compounds may be formed in the plating layer, making the plating layer brittle and reducing the adhesion to the steel sheet. Therefore, the Ca content is preferably 4.000% or less. The Ca content may be 3.000% or less, 2.000% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[Sb: 0-0.500%, Pb: 0-0.500%, Cu: 0-1.000%, Sn: 0-1.000%, Ti: 0-1.000%, Cr: 0-1. 000%, Nb: 0-1.000%, Zr: 0-1.000%, Mn: 0-1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0-1.000%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.500%, Y: 0-0.500%, Sr: 0-0.500%, I n: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, W: 0 to 0.500% and V: 0 to 0.500%]
Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, W, and V may not be contained in the plating layer, but may be present in the plating layer in an amount of 0.0001% or more, 0.001% or more, or 0.01% or more. These elements do not adversely affect the performance of the plated steel sheet as long as they are within a predetermined content range. However, if the content of each element is excessive, the corrosion resistance may be reduced. Therefore, the content of Sb, Pb, La, Ce, B, Y, Sr, In, Co, Bi, P, W, and V is preferably 0.500% or less, and may be, for example, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% or less. Similarly, the contents of Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag and Li are preferably 1.000% or less, and may be, for example, 0.800% or less, 0.500% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

In the plating layer, the remainder other than the above elements consists of 20.0% or more Al and impurities. The Al content may be, for example, 25.0% or more, 30.0% or more, 35.0% or more, 40.0% or more, 45.0% or more, or 50.0% or more. Similarly, the Al content may be, for example, 80.0% or less, 75.0% or less, 70.0% or less, 65.0% or less, or 60.0% or less. Impurities in the plating layer are components that are mixed in due to various factors in the manufacturing process, including raw materials, when the plating layer is manufactured.

[Measurement of chemical composition of plating layer]
The chemical composition of the plating layer is determined as follows. First, the plating layer is peeled off and dissolved from the plated steel sheet using an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet, and the resulting acid solution is measured by ICP (inductively coupled plasma) emission spectroscopy to determine the chemical composition (average composition) of the plating layer. The type of acid is not particularly limited, and may be any acid that can dissolve the plating layer. The chemical composition of the plating layer in this embodiment is the average of measurements performed on three samples.

[(L-L ₀ )/L ₀ ×100≧3]
In an embodiment of the present invention, the morphology of the plating layer is controlled so that, in the cross section of the plating layer, the interface length _L between the plating layer and the base steel sheet and the surface length L 0 of the base steel sheet satisfy the relationship (L - L ₀ ) / L ₀ × 100 ≧ 3, i.e., the interface length L is 3% or more longer than the surface length L ₀ of the base steel sheet. As described above in relation to Fig. 1, by having a shape with larger irregularities such that the interface between the plating layer and the base steel sheet satisfies the relationship (L - L ₀ ) / L ₀ × 100 ≧ 3, during cold working such as bending, the hard plating layer can bite into the base steel sheet from the irregularities at the interface and deform the base steel sheet while proceeding with the cold working. As a result, it is possible to significantly suppress the occurrence of powdering due to bending, etc., that is, it is possible to significantly improve the cold workability of the plated steel sheet. In order to further enhance such an effect, it is preferable to control the interface shape between the coating layer and the base steel sheet to one with greater irregularities, that is, to make (LL ₀ )/L ₀ × 100 a larger value. More specifically, the value of (LL ₀ )/L ₀ × 100 is preferably 4 or more, and may be, for example, 5 or more, 6 or more, 7 or more, or 8 or more. There is no particular upper limit, but the value of (LL ₀ )/L ₀ × 100 may be, for example, 30 or less, 20 or less, 15 or less, 12 or less, or 10 or less.

[Fe-Al phase thickness: 4 to 50 μm]
In an embodiment of the present invention, the plating layer includes an Fe-Al phase, and the thickness of the Fe-Al phase is 4 to 50 μm. In the present invention, the Fe-Al phase refers to a phase having a chemical composition consisting of, in mass%, Fe: 40 to 70%, Al: 30 to 60%, Zn: 0 to 20%, and other elements: less than 3% (i.e., Fe, Al, and Zn: more than 97% in total). By controlling the thickness of the Fe-Al phase contained in the plating layer to 4 μm or more while setting the Fe content in the plating layer to 20.0 mass% or more as described above, the plating layer can be sufficiently alloyed, thereby improving the corrosion resistance after painting of the plated steel sheet. From the viewpoint of further improving the corrosion resistance after painting, the thicker the Fe-Al phase, the more preferable it is, and it may be, for example, 6 μm or more, 8 μm or more, 10 μm or more, 12 μm or more, 14 μm or more, or 16 μm or more. On the other hand, if the Fe-Al phase is too thick, it may lead to excessive hardening of the plating layer and reduce the cold workability of the plated steel sheet. Therefore, the thickness of the Fe-Al phase is set to 50 μm or less, and may be, for example, 40 μm or less, 30 μm or less, 25 μm or less, or 20 μm or less.

[Fe-Al-Si phase]
[ΣT _i /L ₀ ×100≦20]
According to a preferred embodiment of the present invention, in the cross section of the coating layer, the projected length T _i of the Fe-Al-Si phase in the coating layer and the length L ₀ of the surface of the base steel sheet are controlled to satisfy ΣT _i /L ₀ ×100≦20. The Fe-Al-Si phase is an Fe-Al intermetallic compound containing a relatively large amount of Si, and more specifically, in the present invention, the Fe-Al-Si phase refers to a phase having a chemical composition consisting of, in mass%, Fe: 30-70%, Al: 30-60%, Si: 3-20%, and other elements: less than 3%. Therefore, if the Fe-Al-Si phase is present in a relatively large amount in the coating layer, galvanic corrosion may occur between the Fe-Al phase and the Fe-Al phase. Therefore, in a preferred embodiment of the present invention, the Fe—Al—Si phase is dispersed in the coating layer, that is, the projected length T _i of the Fe—Al—Si phase in the coating layer and the length L ₀ of the surface of the base steel sheet are controlled to satisfy ΣT _i /L ₀ × 100≦20, thereby making it possible to further improve the post-painting corrosion resistance of the coated steel sheet.

Fig. 2 is a cross-sectional schematic diagram of a plated steel sheet according to a preferred embodiment of the present invention, showing the projected length T _i of the Fe-Al-Si phase and the length L ₀ of the surface of the base steel sheet. Referring to Fig. 2, the plated steel sheet 1 includes a base steel sheet 2 and a plated layer 3 formed on the surface of the base steel sheet 2, and the plated layer 3 includes an Fe-Al phase 4 and an Fe-Al-Si phase 5. Here, it can be understood that the sum ΣT _i of the projected lengths T _i of the Fe-Al-Si phases 5 projected onto the surface of the base steel sheet 2 (ΣT _i = T ₁ + T ₂ in Fig. 2) and the length L ₀ of the surface of the base steel sheet 2 satisfy ΣT _i /L ₀ × 100 ≦ 20 (i.e., the projection ratio of the Fe-Al-Si phase is 20% or less), and therefore the Fe-Al-Si phase 5 is present in a dispersed state in the Fe-Al phase 4. As is clear from the projected length _T1 in Fig. 2, when the projected lengths of a plurality of Fe-Al-Si phases 5 partially overlap, the entire projected length including the overlapping parts is determined as one projected length. Unlike the case shown in Fig. 2, if the Fe-Al-Si phases 5 are present in a layered form in the Fe-Al phase 4, it is considered that when galvanic corrosion occurs at the contact portion between the layered Fe-Al-Si phases 5 and the Fe-Al phase 4, the corrosion will progress along the contact interface. As a result, the corrosion resistance of the plating layer 3 will be significantly reduced. In contrast, in the plated steel sheet 1 according to a preferred embodiment of the present invention, the Fe-Al-Si phases 5 are dispersed in the Fe-Al phase 4 so as to satisfy ΣT _i /L ₀ × 100≦20. Therefore, even if galvanic corrosion occurs at the contact area between one or more Fe-Al-Si phases 5 and the Fe-Al phase 4 present therearound, the corrosion will not progress from the corrosion at the contact area to other Fe-Al-Si phases 5, and therefore the corrosion resistance of the plated steel sheet 1, particularly the corrosion resistance after painting, can be further improved.

By controlling the Si content in the plating layer, the value of ΣT _i /L ₀ ×100, which is the projection ratio of the Fe-Al-Si phase, can be reliably reduced. For example, by controlling the Si content in the plating layer to 1.0% or less, it is possible to reliably satisfy ΣT _i /L ₀ ×100≦20. From the viewpoint of further enhancing the corrosion resistance improvement effect, the lower the value of ΣT _i /L ₀ ×100, the more preferable it is, and it may be, for example, 15 or less, 10 or less, 5 or less, 3 or less, 2 or less, or 1 or less. From the viewpoint of further improving the corrosion resistance, the Fe-Al-Si phase 5 may not be present in the plating layer. That is, the lower limit of ΣT _i /L ₀ ×100 may be 0. Although not particularly limited, for example, the value of ΣT _i /L ₀ ×100 may be 0.1 or more, 0.2 or more, or 0.3 or more.

[Surface coverage of Mg-containing phase: 20 to 100%]
According to another preferred embodiment of the present invention, the Mg content in the plating layer is 0.3-10.0%, and in this connection, the plating layer further contains an Mg-containing phase, and the surface coverage of the Mg-containing phase is controlled to 20-100% in the cross section of the plating layer. In the present invention, the Mg-containing phase refers to a phase having a chemical composition consisting of, in mass%, Mg: 0.5-90%, Al: 10-99.5%, O: 0-70%, Fe: 0-3%, and other elements: less than 3%. As is clear from the chemical composition, the Mg-containing phase does not include the _MgZn2 phase described below.

FIG. 3 is a schematic cross-sectional view of a plated steel sheet according to another preferred embodiment of the present invention, and illustrates the surface coverage of the Mg-containing phase. Referring to FIG. 3, the plated steel sheet 1, like the plated steel sheet 1 in FIGS. 1 and 2, includes a base steel sheet 2 and a plated layer 3 formed on the surface of the base steel sheet 2, and the plated layer 3 includes an Fe-Al phase 4. In FIG. 3, the plated layer 3 further includes an Mg-containing phase 6 in its surface portion. Here, it can be understood that the sum ΣM _i of the lengths M _i of the Mg-containing phases 6 (ΣM _i = M ₁ + M ₂ + M ₃ in FIG. 3) and the length L ₀ of the surface of the base steel sheet 2 satisfy ΣM _i /L ₀ × 100≧20, that is, the surface coverage of the Mg-containing phase satisfies 20% or more. By controlling the surface coverage of the Mg-containing phase to 20% or more and allowing a relatively large amount of Mg to be present on the surface of the plated layer, the reaction can be promoted by the action of Mg during chemical conversion treatment, and adhesion of the chemical conversion coating to the plated steel sheet can be improved. From the viewpoint of further enhancing the effect of improving chemical conversion treatability, the higher the surface coverage of the Mg-containing phase, the more preferable, and for example, it may be 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, or 80% or more. The upper limit is not particularly limited, and the surface coverage of the Mg-containing phase may be 100%. For example, the surface coverage of the Mg-containing phase may be 95% or less or 90% or less. In order to increase the surface coverage of the Mg-containing phase, it is preferable to increase the Mg content in the plating layer. More specifically, the Mg content in the plating layer is preferably 0.3% or more, and more preferably 0.6% or more. However, since the surface coverage of the Mg-containing phase does not depend only on the Mg content, the Mg content in the plating layer may be appropriately determined according to the desired surface coverage while considering the manufacturing conditions, etc.

[Area ratio of MgZn2 _phase : less than 10%]
In the embodiment of the present invention, in relation to the upper limit of the Zn content in the plating layer being 30.0%, the MgZn ₂ phase may be formed in the plating layer in an area percentage range of less than 10%. The MgZn ₂ phase may or may not be included in the plating layer. When the MgZn 2 phase is included in the plating layer, it may contribute to improving the sacrificial corrosion protection. The area percentage of the MgZn ₂ phase may be, for example, 9% or less, 7% or less, 5% or less, or 3% or less. Similarly, the area percentage of _{the MgZn 2 phase} may be 0%, or, for example, 1% or more, or 2% or more.

[Analysis of plating layer]
The plating layer is analyzed as follows. First, a 15 mm x 20 mm sample is taken from the surface of the plated steel sheet so that the cross section of the plating layer can be observed, and the taken sample is embedded in resin and then polished. Next, for the obtained mirror-polished sample, a backscattered electron image (BSE image) is obtained using a scanning electron microscope with an electron probe microanalyzer (SEM-EPMA) in a field of view of 80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction, and the plating layer is identified from the BSE image. Next, the composition of each phase in the identified plating layer is point analyzed. From the obtained composition, the Fe-Al phase (Fe: 40-70%, Al: 30-60%, Zn: 0-20%, and other elements: less than 3%), the Fe-Al-Si phase (Fe: 30-70%, Al: 30-60%, Si: 3-20%, and other elements: less than 3%), the Mg-containing phase (Mg: 0.5-90%, Al: 10-99.5%, O: 0-70%, Fe: 0-3%, and other elements: less than 3%), and the MgZn ₂ phase are identified. The specific measurement conditions of the EPMA in the above field of view are as follows.
Apparatus: JXA-8500 manufactured by JEOL Ltd.
Acceleration voltage: 15 kV
Irradiation current: 5×10 ^-7 A
Irradiation time: 50 ms

(L-L ₀ )/L ₀ ×100 is determined as follows. First, the mirror-polished sample obtained above is observed with an SEM in a visual field of 80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction to obtain a BSE image. The BSE image is measured using the "Analyze" function of the image analysis software "ImageJ" to measure the interface length between the coating layer and the base steel sheet (interface length L between the coating layer and the base steel sheet shown in FIG. 1). The above operation is performed for five visual fields, and the average value is calculated to determine the interface length L. Next, (L-L 0 )/L 0 ×100 is determined from the obtained interface length L and the length _{L 0} of the surface of the corresponding base steel sheet, i.e., the length of the long side of the observation visual field: ₁₀₀ μm. The resolution of the SEM image is 2560 × 1920. L ₀ is measured using "Find edge" in the "Process" function of the image analysis software "ImageJ", binarizing with the "Binary" function, and then reading "Perim." with the "Measure" function in "Analyze".

The thickness of the Fe-Al phase is determined as follows. First, the thickness of the Fe-Al phase identified above is measured at five different points in the field of view using the "Analyze" function of the image analysis software "ImageJ", and then the thickness of the Fe-Al phase is determined by averaging the thicknesses measured at the five points.

ΣT _i /L ₀ ×100 (projection ratio of Fe-Al-Si phase) is determined as follows. First, the Fe-Al-Si phase identified above is projected onto the surface of the base steel sheet using the image analysis software "ImageJ", and the sum ΣT _i of the projected lengths T _i of each Fe-Al-Si phase (T ₁ +T ₂ in FIG. 2) is calculated. Specifically, a straight line is drawn in the horizontal direction of each Fe-Al-Si phase using the toolbar "Straight" in ImageJ, and T _i is measured by reading the value displayed in "Length" on the toolbar. Next, ΣT _i /L ₀ ×100 (projection ratio of Fe-Al-Si phase) is determined from the calculated ΣT _i and the length L ₀ of the corresponding surface of the base steel sheet (length of the long side of the observation field: 100 μm).

The surface coverage of the Mg-containing phase is determined as follows. First, the sum ΣM i of the lengths M _i of the Mg-containing phases present on the surface of the coating layer among the Mg-containing phases identified above _{(ΣM i =} M ₁ + M ₂ + M ₃ in FIG. 3) is calculated using the "Analyze" function of the image analysis software "ImageJ". Specifically, when a horizontal line is drawn on both ends of each Mg-containing phase using the toolbar "Straight" in ImageJ, M _i is measured by reading the value displayed in "Length" on the toolbar. Next, ΣM _i /L ₀ ×100 (surface coverage of the Mg-containing phase) is determined from the calculated ΣM _i and the corresponding surface length L ₀ of the base steel sheet (length of the long side of the observation field: 100 μm).

The area ratio of the MgZn ₂ phase is measured from the element distribution image of the mapping image obtained from the above sample by the image analysis software "ImageJ". Specifically, the region containing 25 to 45 at% Mg and 50 to 75 at% Zn (Mg + Zn: 90 to 100 at%) in the element mapping is binarized by the "Binary" function of "ImageJ" and the area ratio is measured by the "Analyze" function.

The plating layer may be any plating layer having the above chemical composition, an Fe-Al phase, an Fe-Al-Si phase, an Mg-containing phase, and/or an MgZn ₂ phase, but is not particularly limited thereto. For example, it may be an alloyed hot-dip plating layer.

[Preferable chemical composition of base steel sheet]
As described above, the present invention aims to provide a plated steel sheet having an Al-containing plating layer and improved corrosion resistance and cold workability after painting, and the object is achieved by optimizing the chemical composition of the plating layer, controlling the thickness of the Fe-Al phase contained in the plating layer to within a range of 4 to 50 μm, and controlling the interface shape between the plating layer and the base steel sheet to satisfy the relationship (L-L ₀ )/L ₀ ×100≧3. Therefore, it is clear that the chemical composition of the base steel sheet itself is not an essential technical feature for achieving the object of the present invention. Hereinafter, a preferred chemical composition of the base steel sheet used in the plated steel sheet according to the embodiment of the present invention will be described in detail, but these descriptions are intended to be merely examples of the preferred chemical composition of the base steel sheet, and are not intended to limit the present invention to one using a base steel sheet having such a specific chemical composition.

In an embodiment of the present invention, for example, the base steel plate contains, in mass%,
C: 0.01-0.50%,
Si: 0.001 to 3.000%,
Mn: 0.10-3.00%,
Al: 0.0002-2.000%,
P: 0.100% or less,
S: 0.1000% or less,
N: 0.0100% or less,
Nb: 0 to 0.15%,
Ti: 0 to 0.15%,
V: 0 to 0.15%,
Mo: 0-1.0%,
Cr: 0-1.0%,
Cu: 0 to 1.0%,
Ni: 0-1.0%,
B: 0 to 0.0100%,
W: 0-1.000%,
Hf: 0 to 0.050%,
Mg: 0 to 0.050%,
Zr: 0 to 0.050%,
Ca: 0-0.010%,
REM: 0-0.30%,
It is preferable that the chemical composition is Ir: 0 to 1.000%, and the balance: Fe and impurities. Each element will be described in more detail below.

[C: 0.01-0.50%]
C is an element that inexpensively increases tensile strength and is an important element for controlling the strength of steel. In order to fully obtain such an effect, the C content is preferably 0.01% or more. The C content may be 0.05% or more, 0.10% or more, 0.15% or more, 0.20% or more, 0.30% or more, or 0.35% or more. On the other hand, excessive C content may cause a decrease in elongation. For this reason, the C content is preferably 0.50% or less. The C content may be 0.45% or less, or 0.40% or less.

[Si: 0.001 to 3.000%]
Si acts as a deoxidizer and is an element that suppresses the precipitation of carbides during the cooling process during cold-rolled sheet annealing. In order to fully obtain such effects, the Si content is preferably 0.001% or more. The Si content may be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, excessive Si content may lead to an increase in steel strength and a decrease in elongation. For this reason, the Si content is preferably 3.000% or less. The Si content may be 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.

[Mn: 0.10-3.00%]
Mn is an element that enhances the hardenability of steel and is effective in increasing strength. In order to fully obtain such effects, the Mn content is preferably 0.10% or more. The Mn content may be 0.30% or more, 0.50% or more, 1.00% or more, or 1.30% or more. On the other hand, excessive Mn content may increase the steel strength and decrease the elongation. For this reason, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.

[Al: 0.0002-2.000%]
Al acts as a deoxidizer for steel and is an element that has the effect of improving the soundness of steel. In order to fully obtain such an effect, the Al content is preferably 0.0002% or more. The Al content may be 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if Al is contained excessively, coarse Al oxides may be generated and the elongation of the steel sheet may decrease. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[P: 0.100% or less]
P is an element that segregates at grain boundaries and promotes embrittlement of steel. Since the lower the P content, the better, it is ideally 0%. However, excessive reduction in the P content may lead to a significant increase in costs. For this reason, the P content may be 0.0001% or more, or may be 0.001% or more, or 0.005% or more. On the other hand, excessive inclusion of P may lead to embrittlement of steel due to grain boundary segregation as described above. Therefore, the P content is preferably 0.100% or less. The P content may be 0.050% or less, 0.030% or less, or 0.010% or less.

[S: 0.1000% or less]
S is an element that generates nonmetallic inclusions such as MnS in steel, which leads to a decrease in the ductility of steel parts. The lower the S content, the better, so ideally it is 0%. However, excessive reduction in the S content may lead to a significant increase in costs. For this reason, the S content may be 0.0001% or more, 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content may lead to the occurrence of cracks originating from nonmetallic inclusions during cold forming. Therefore, the S content is preferably 0.1000% or less. The S content may be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[N: 0.0100% or less]
N is an element that forms coarse nitrides in the steel sheet and reduces the workability of the steel sheet. Since the lower the N content, the more preferable it is, the ideal N content is 0%. However, excessive reduction in the N content may lead to a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, thereby reducing the workability of the steel sheet. Therefore, the N content is preferably 0.0100% or less. The N content may be 0.0080% or less, or 0.0050% or less.

The preferred basic chemical composition of the base steel plate is as described above. Furthermore, the base steel plate may contain, as necessary, one or more elements selected from the group consisting of Nb: 0-0.15%, Ti: 0-0.15%, V: 0-0.15%, Mo: 0-1.0%, Cr: 0-1.0%, Cu: 0-1.0%, Ni: 0-1.0%, B: 0-0.0100%, W: 0-1.000%, Hf: 0-0.050%, Mg: 0-0.050%, Zr: 0-0.050%, Ca: 0-0.010%, REM: 0-0.30%, and Ir: 0-1.000%, in place of a portion of the remaining Fe. Each of these elements may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more.

The remainder of the base steel plate, other than the above elements, consists of Fe and impurities. Impurities in base steel plate are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel plate is industrially manufactured.

The chemical composition of the base steel plate may be measured by a general analytical method. For example, the chemical composition of the base steel plate may be measured by first removing the plating layer by mechanical grinding, and then using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on the cutting chips in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece may be obtained from the vicinity of the 1/2 position of the plate thickness of the base steel plate, and the composition may be identified by measuring it under conditions based on a calibration curve created in advance using a Shimadzu ICPS-8100 or similar (measuring device). C and S, which cannot be measured by ICP-AES, may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

[Thickness of base steel plate]
The thickness of the base steel plate is not particularly limited, and may be, for example, 0.2 mm or more, 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. Similarly, the thickness of the base steel plate may be, for example, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

<Method of manufacturing plated steel sheet>
Next, a preferred method for producing a plated steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for producing a plated steel sheet according to an embodiment of the present invention, and is not intended to limit the plated steel sheet to one produced by the production method described below.

The plated steel sheet according to an embodiment of the present invention can be manufactured by, for example, carrying out a casting process in which molten steel with an adjusted chemical composition is cast to form a steel slab, a hot rolling process in which the steel slab is hot rolled to obtain a hot rolled steel sheet, a coiling process in which the hot rolled steel sheet is coiled, a cold rolling process in which the coiled hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet, a pretreatment process, an annealing process in which the pretreated cold rolled steel sheet is annealed, a cooling process in which the annealed cold rolled steel sheet is cooled, and a plating process in which a plating layer is formed on the obtained base steel sheet. Alternatively, the cold rolling process may be carried out directly after pickling without coiling after the hot rolling process. Each process will be described in detail below.

[Casting process]
The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a method such as ordinary continuous casting or casting by an ingot method.

[Hot rolling process]
The cast steel slab can be hot-rolled to obtain a hot-rolled steel sheet. The hot rolling step is performed by hot-rolling the cast steel slab directly or after cooling it once and then reheating it. When reheating is performed, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling step, rough rolling and finish rolling are usually performed. The temperature and reduction of each rolling step can be appropriately determined according to the desired metal structure and plate thickness. For example, the end temperature of the finish rolling may be 900 to 1050°C, and the reduction of the finish rolling may be 10 to 50%.

[Winding process]
The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature can be appropriately determined depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be subjected to a predetermined heat treatment by recoiling before or after coiling. Alternatively, the coiling step may be omitted, and the hot-rolled steel sheet may be pickled after the hot-rolling step and then subjected to the cold-rolling step described below.

[Cold rolling process]
After pickling or the like is performed on the hot-rolled steel sheet, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The rolling reduction in the cold rolling can be appropriately determined according to the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling process, the steel sheet may be cooled to room temperature, for example, by air cooling.

[Pretreatment process]
Next, a predetermined pretreatment process may be performed before annealing the cold-rolled steel sheet. Such a pretreatment process may include a degreasing process. The degreasing process may include passing an electric current through the cold-rolled steel sheet in a solution having a pH of 8.0 or more (electrolysis process). The current density during the current passing may be 1.0 to 8.0 A/ ^dm2 , and the current passing time may be 5 to 10 seconds.

[Annealing process]
Next, the obtained cold-rolled steel sheet is annealed. The annealing step includes heating the cold-rolled steel sheet to a temperature of 780 to 900°C in an atmosphere with a dew point of -10 to 10°C and holding the temperature for 10 to 300 seconds. By carrying out the annealing step under such conditions, the surface layer of the cold-rolled steel sheet can be appropriately decarburized. In this case, the reaction between the plating layer and the base steel sheet is promoted during the alloying treatment in the subsequent plating step, that is, it is possible to increase the alloying rate. As a result, it is possible to realize an interface shape with larger irregularities such that the interface length _L between the plating layer and the base steel sheet and the corresponding surface length L 0 of the base steel sheet satisfy the relationship (L - L ₀ ) / L ₀ × 100 ≧ 3, and thereby it is possible to significantly improve the cold workability of the plated steel sheet.

If the dew point is lower than -10°C, the annealing temperature is lower than 780°C, and/or the annealing time is shorter than 10 seconds, the decarburization of the surface layer of the cold-rolled steel sheet is insufficient, and a sufficient alloying rate cannot be obtained during the alloying treatment of the coating layer. As a result, it becomes impossible to realize an interface shape that satisfies the relationship (LL ₀ )/L ₀ × 100 ≧ 3 between the coating layer and the base steel sheet. On the other hand, if the dew point is higher than 10°C, the heating temperature is higher than 900°C, and/or the annealing time is longer than 300 seconds, an outer oxide layer is formed on the surface of the base steel sheet, which may deteriorate the coating property or may deteriorate the strength of the finally obtained coated steel sheet due to excessive decarburization. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (for example, 3% hydrogen and the balance of nitrogen).

[Cooling process]
The cold-rolled steel sheet whose surface layer has been decarburized in the annealing process needs to be appropriately cooled in the subsequent cooling process in order to obtain a desired surface layer structure. Specifically, the cooling process includes cooling from the heating temperature (annealing temperature) in the annealing process to a controlled temperature of 500 to 750° C. at an average cooling rate of 5° C./s or more. This will be described in detail below.

In general, the annealed cold-rolled steel sheet is then cooled to a temperature below 500°C, for example, to a temperature of about 200°C, and then reheated to be plated. However, when the steel sheet undergoes such a temperature history, the metal structure that has been austenitized in the annealing process is transformed into a structure such as bainite or martensite. Therefore, in the subsequent plating process, the metal structure such as bainite or martensite is alloyed with the plating layer. However, since the alloying speed between these metal structures and the plating layer is relatively slow, the finally obtained plated steel sheet cannot realize an interface shape that satisfies the relationship (L- _L0 )/ _L0 x 100 ≥ 3 between the plating layer and the base steel sheet. Therefore, in the cooling process of the present manufacturing method, it is extremely important to immerse the metal structure of the cold-rolled steel sheet, the surface layer of which has been decarburized in the annealing process, in a plating bath while still containing a large amount of austenite phase, and directly alloy the austenite phase with the plating layer. In this regard, in the present cooling step, the metal structure of the cold-rolled steel sheet can be maintained in a state containing a larger amount of austenite phase by cooling from the annealing temperature to a controlled temperature of 500 to 750° C. at an average cooling rate of 5° C./s or more. As a result, it becomes possible to achieve an alloying rate sufficient for directly alloying the austenite phase and the plating layer in the subsequent plating step to realize a desired interface shape.

Although it is not intended to be bound by any particular theory, it is believed that by increasing the alloying rate by combining decarburization and the austenite phase, unevenness in the alloying rate occurs between the area where the austenite grain boundary is present and the area where the austenite grain boundary is not present, and such unevenness in the alloying rate causes the formation of an uneven shape at the interface between the coating layer and the base steel sheet. If the control temperature is less than 500°C, the austenite phase transforms into bainite or martensite, making it impossible to achieve a sufficient alloying rate in the subsequent coating process. Also, if the average cooling rate from the annealing temperature to the control temperature of 500 to 750°C is less than 5°C/s, the transformation to ferrite becomes significant, and similarly, it becomes impossible to achieve a sufficient alloying rate in the subsequent coating process. As a result, in either case, it becomes impossible to realize an interface shape that satisfies the relationship (L-L ₀ )/L ₀ ×100≧3 between the coating layer and the base steel sheet. On the other hand, if the control temperature exceeds 750°C, the temperature becomes higher than the temperature suitable for the subsequent coating process, and the desired coating layer may not be obtained. From the viewpoint of realizing an interface shape with larger irregularities, the higher the average cooling rate from the annealing temperature to the control temperature of 500 to 750° C., the more preferable, and for example, the rate is preferably 15° C./s or more. Although the upper limit is not necessarily limited, the average cooling rate is preferably, for example, 30° C./s or less.

[Plating process]
Next, in the plating step, a plating layer is formed on at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet). More specifically, the plating step is performed by immersing the cold-rolled steel sheet cooled to the above-mentioned controlled temperature in a plating bath (plating bath temperature: for example, 680 to 750°C) having a predetermined chemical composition while maintaining a state in which the austenite phase is contained in a larger amount, and then heat treating it for 0.5 to 20 seconds at an alloying temperature of 680 to 750°C. By performing the alloying treatment under such conditions, the plating layer is appropriately alloyed so that the thickness of the Fe-Al phase is 4 μm or more, and a sufficient alloying speed can be realized based on the combination of decarburization and austenite phase. As a result, it is possible to realize an interface shape with larger irregularities such that the interface length _L between the plating layer and the base steel sheet and the corresponding surface length L 0 of the base steel sheet satisfy the relationship (L - L ₀ ) / L ₀ × 100 ≧ 3, thereby making it possible to significantly improve the cold workability of the plated steel sheet.

If the alloying temperature is lower than 680°C, the plating layer solidifies in a state where the alloying is insufficient, so that the Fe content in the plating layer decreases and/or the desired Fe-Al phase thickness cannot be obtained. As a result, the corrosion resistance after painting of the plating steel sheet decreases. Also, if the alloying treatment time is shorter than 0.5 seconds, the alloying of the plating layer is insufficient, so that the interface between the plating layer and the base steel sheet cannot be made into an uneven shape and/or the desired Fe-Al phase thickness cannot be obtained. As a result, the cold workability and/or the corrosion resistance after painting of the plating steel sheet decreases. On the other hand, if the alloying temperature is higher than 750°C or the alloying treatment time is longer than 20 seconds, the alloying of the plating layer progresses excessively, so that the interface between the plating layer and the base steel sheet has a flatter shape with less unevenness, and the finally obtained plating steel sheet may not satisfy the relationship (L-L ₀ )/L ₀ ×100≧3. In this case, the cold workability of the plating steel sheet decreases. In order to ensure the desired alloying, the alloying treatment time is preferably set to 5 to 20 seconds.

The plating process is carried out, for example, by hot-dip plating. The plating process is not limited to hot-dip plating, and may be electroplating, vapor deposition plating, thermal spraying, cold spraying, or the like. Other conditions of the plating process may be appropriately set in consideration of the thickness and adhesion amount of the plating layer. For example, after immersing a cold-rolled steel sheet in a plating bath, it is pulled up, and immediately _N2 gas or air is blown onto it by a gas wiping method, and then cooled, so that the adhesion amount of the plating layer can be adjusted within a predetermined range, for example, within a range in which the thickness of the Fe-Al phase is 4 to 50 μm.

[Cooling after plating]
Finally, the base steel sheet to which the plating layer is attached is cooled to obtain a plated steel sheet according to an embodiment of the present invention. The cooling after plating is not particularly limited and can be performed under any appropriate conditions known to those skilled in the art. For example, the cooling after plating can be performed at an average cooling rate of 10°C/s or more. The cooling stop temperature is also not particularly limited and may be appropriately set within the range of, for example, 100 to 350°C.

According to this manufacturing method, it is possible to manufacture a plated steel sheet having a plating layer in which the chemical composition of the plating layer is optimized within a predetermined range, i.e., by mass %, Fe: 20.0 to 55.0%, Mg: 0 to 10.0%, Si: 0 to _10.0 %, and Al: _20.0 % or more, the thickness of the Fe-Al phase contained in the plating layer is controlled within a range of 4 to 50 μm, and the interface shape between the plating layer and the base steel sheet is controlled to satisfy the relationship of (L-L 0 )/L 0 × 100≧3. Therefore, due to the appropriately alloyed Fe-Al phase in the plating layer, sufficient corrosion resistance after painting can be ensured and cold workability can be improved. In addition, by controlling the interface shape between the plating layer and the base steel sheet to a shape with larger irregularities, even when cold working such as bending is performed, the hard plating layer can bite into the base steel sheet from the irregularities at the interface and deform the base steel sheet while the cold working proceeds. As a result, it is possible to significantly suppress the occurrence of powdering due to bending or the like, that is, it is possible to significantly improve the cold workability of the plated steel sheet. In addition, by appropriately controlling the Si content in the plated layer, the Fe-Al-Si phase can be dispersed and present in the Fe-Al phase, thereby making it possible to further improve the corrosion resistance after painting of the plated steel sheet. Furthermore, by appropriately controlling mainly the Mg content in the plated layer, it is possible to increase the surface coverage of the plated layer by the Mg-containing phase, thereby making it possible to significantly improve the chemical conversion treatability of the plated steel sheet. Therefore, according to such a plated steel sheet, it is possible to realize excellent corrosion resistance after painting and cold workability compared to conventional plated steel sheets. Therefore, it is possible to contribute to the development of industry through improved productivity in the use of plated steel sheets for automobiles and building materials.

The present invention will be described in more detail below with reference to examples. However, the following examples are merely examples of the present invention, and the present invention is in no way limited to these examples. It goes without saying that the present invention can be modified as desired without departing from the gist of the present invention.

In the following examples, plated steel sheets according to embodiments of the present invention were manufactured under various conditions, and the properties of the manufactured plated steel sheets were investigated.

First, molten steel was cast by a continuous casting method to form a steel slab having a chemical composition consisting of, in mass%, C: 0.20%, Si: 0.012%, Mn: 1.30%, Al: 0.030%, P: 0.005%, S: 0.0020%, and N: 0.0030%, with the balance being Fe and impurities. The steel slab was once cooled, then reheated to 1200 ° C. and hot rolled, and then coiled at a temperature of 600 ° C. or less. The hot rolling was performed by performing rough rolling and finish rolling, the finish rolling end temperature was 900 to 1050 ° C., and the rolling reduction ratio of the finish rolling was 30%. Next, the obtained hot-rolled steel sheet was pickled, and then cold rolled at a rolling reduction ratio of 50% to obtain a cold-rolled steel sheet having a sheet thickness of 0.8 mm. Next, the obtained cold-rolled steel sheet was subjected to a pretreatment (degreasing treatment) in which a current was passed through the sheet in a solution of pH 9.2 at a current density of 5.0 A/dm ² for 8 seconds.

Next, each cold-rolled steel sheet was cut into a size of 100 mm x 200 mm, and then annealed under the conditions shown in Table 1 (annealing atmosphere: hydrogen 3% and nitrogen balance). Next, the cut steel sheet sample was cooled from the annealing temperature to the control temperature at the average cooling rate shown in Table 1, and then immersed in a hot-dip galvanizing bath (galvanizing bath temperature: 680 to 750 ° C) having a predetermined bath composition, and alloyed under the conditions shown in Table 1. The coating weight was adjusted by pulling up the steel sheet sample after immersion in the galvanizing bath and wiping with _N2 gas. Finally, the base steel sheet to which the coating layer was attached was cooled at an average cooling rate of 10 ° C / s or more to obtain a coated steel sheet in which a coating layer was formed on both sides of the base steel sheet.

The physical properties and characteristics of the resulting plated steel sheets were measured and evaluated using the following methods.

[Chemical composition analysis of plating layer]
The chemical composition of the plating layer was determined by immersing a sample cut to 30 mm x 30 mm in a 10% HCl aqueous solution containing 0.04% IBIT 710K (manufactured by Asahi Chemical Industry Co., Ltd.) as an inhibitor, pickling the plating layer to remove it, and then measuring the plating components dissolved in the aqueous solution by ICP emission spectroscopy. The results are shown in Table 1.

[Evaluation of cold workability]
The cold workability was evaluated as follows. First, a 100×50 mm×0.8 mm sample of the plated steel sheet was subjected to a 90° bending test with R=2 mm, then ultrasonically cleaned, and the sample mass was measured. The difference from the sample mass before the 90° bending test was measured as the amount of powdering, and the cold workability, particularly the powdering resistance, was evaluated as follows.
AAA: 6mg or less AA: More than 6 to 12mg
A: More than 12 to 24 mg
B: More than 24 mg

[Evaluation of corrosion resistance after painting]
The corrosion resistance after painting was evaluated as follows. First, a 50 mm x 100 mm plated steel sheet sample was treated with Zn phosphate (SD5350 system: Nippon Paint Industrial Coating Co., Ltd. standard), then electrocoated (PN110 Powernics Gray: Nippon Paint Industrial Coating Co., Ltd. standard) to 20 μm, and baked at 150° C. for 20 minutes. Next, a cut was introduced in the center of the sample that reached the base steel (base steel sheet). Next, a cyclic corrosion test according to JASO (M609-91) was performed for 180 cycles to measure the paint film blister width, and the corrosion resistance after painting was evaluated as follows.
AAA: 2mm or less AA: More than 2 to 3mm
A: More than 3 to 4 mm
B: More than 4 mm

[Evaluation of chemical conversion treatment properties]
The chemical conversion treatability was evaluated as follows. First, a 50 mm x 100 mm sample of plated steel sheet was treated with zinc phosphate (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.) to form a chemical conversion coating. Next, the sample surface was observed with a secondary electron image of an SEM, and the area ratio of the portion where the chemical conversion coating was not formed, generally called "blank", was measured. The chemical conversion treatability of the plated steel sheet was evaluated according to the area ratio of blank, using the following evaluation criteria.
AA: Blank area rate 0-5%
A: Scale area ratio: 5% to 15%
B: Blank area ratio over 15%

　Steel sheets with cold workability ratings of AAA, AA, and A and corrosion resistance after painting ratings of AAA, AA, and A were evaluated as having improved corrosion resistance after painting and cold workability. The results are shown in Table 1.

With reference to Table 1, in Comparative Examples 30 and 31, the Mg and Si contents in the plating layers were high, and therefore it is believed that a sufficient alloying rate could not be obtained during the alloying treatment of the plating layers. As a result, the value of (L-L ₀ )/L ₀ ×100 at the interface with the base steel sheet was less than 3, i.e., the interface with the base steel sheet had a flatter shape with fewer irregularities, and the cold workability was deteriorated. In Comparative Example 32, the annealing temperature was low, and therefore it is believed that decarburization of the surface layer of the cold-rolled steel sheet was insufficient, and therefore it is believed that a sufficient alloying rate could not be obtained during the alloying treatment of the plating layers. As a result, the value of (L-L ₀ )/L ₀ ×100 was less than 3, and the cold workability was deteriorated. In Comparative Example 33, the annealing time was short, and therefore it is believed that decarburization of the surface layer of the cold-rolled steel sheet was insufficient, and therefore it is believed that a sufficient alloying rate could not be obtained during the alloying treatment of the plating layers. As a result, the value of (L-L ₀ )/L ₀ ×100 was less than 3, and the cold workability was deteriorated. In Comparative Example 34, the dew point in the annealing step was low, so decarburization of the surface layer of the cold-rolled steel sheet was insufficient, and it is considered that a sufficient alloying rate could not be obtained during the alloying treatment of the plating layer. As a result, the value of (L- _L0 )/ _L0 x 100 was less than 3, and the cold workability was deteriorated. In Comparative Example 35, the average cooling rate from the annealing temperature to the control temperature of 500 to 750°C was slow, so the transformation from the austenite phase to ferrite was significant in the metal structure of the cold-rolled steel sheet, and it is considered that a sufficient alloying rate could not be obtained in the subsequent plating step. As a result, the value of (L- _L0 )/ _L0 x 100 was less than 3, and the cold workability was deteriorated.

In Comparative Examples 36 and 37, the controlled temperature in the annealing process was low, so that the transformation from the austenite phase to bainite or martensite became significant in the metal structure of the cold-rolled steel sheet, and it is considered that a sufficient alloying rate could not be obtained in the subsequent plating process. As a result, the value of (L-L ₀ )/L ₀ × 100 was less than 3, and the cold workability was deteriorated. In Comparative Example 38, the alloying temperature of the plating layer was low, so that the plating layer solidified in a state where the alloying was insufficient. As a result, the Fe content in the plating layer was reduced, and the desired Fe-Al phase thickness could not be obtained, and the corrosion resistance after painting was deteriorated. In Comparative Example 39, the alloying temperature of the plating layer was high, so that the alloying of the plating layer proceeded excessively. As a result, the value of (L-L ₀ )/L ₀ × 100 was less than 3, that is, the interface with the base steel sheet became flatter with fewer irregularities, and the cold workability was deteriorated. In Comparative Example 40, the alloying treatment time of the plating layer was short, so that the alloying of the plating layer was insufficient, and the interface between the plating layer and the base steel sheet could not be formed into an uneven shape, that is, the value of (L-L ₀ )/L ₀ ×100 was less than 3, and furthermore, the desired Fe-Al phase thickness could not be obtained. As a result, the cold workability and the corrosion resistance after painting were deteriorated. In Comparative Example 41, it is considered that the alloying of the plating layer progressed excessively because the alloying treatment time of the plating layer was long. As a result, the value of (L-L ₀ )/L ₀ ×100 was less than 3, and the cold workability was deteriorated. In Comparative Example 42, the thickness of the Fe-Al phase was too thick, so that the cold workability of the plated steel sheet was deteriorated due to excessive hardening of the plating layer.

In contrast to this, in the plated steel sheets according to all the examples, the chemical composition of the plating layer was optimized within a predetermined range, i.e., Fe: 20.0-55.0%, Mg: 0-10.0%, Si: 0-10.0%, and Al: 20.0% or more by mass %, the thickness of the Fe-Al phase contained in the plating layer was controlled within a range of 4-50 μm, and further, the interface shape between the plating layer and the base steel sheet was controlled to satisfy the relationship of (L-L ₀ )/L ₀ ×100≧3, whereby both the post-painting corrosion resistance and the cold workability of the obtained plated steel sheets could be significantly improved. In particular, in Examples 8 to 11, in which the value of (L-L ₀ )/L ₀ was controlled to 5 or more, the cold workability was evaluated as AA, and the cold workability was further improved. Similarly, in Examples 12 to 29, in which the value of (L-L ₀ )/L ₀ was controlled to 7 or more, the cold workability was evaluated as AAA, and the cold workability was further improved. In addition, in Examples 14 to 29 in which the thickness of the Fe-Al phase was 12 μm or more and ΣT _i /L ₀ × 100 was controlled to 1 or less (i.e., the projection rate of the Fe-Al-Si phase was 1% or less), the corrosion resistance after painting was evaluated as AAA, and very high corrosion resistance after painting was achieved. In addition, in Examples 8 to 11 in which the surface coverage of the Mg-containing phase was controlled to 20% or more, the chemical conversion treatability was evaluated as A, and similarly, in Examples 12 to 24 and 26 to 29 in which the surface coverage of the Mg-containing phase was controlled to 60% or more, the chemical conversion treatability was evaluated as AA, and very high chemical conversion treatability was achieved.

1 Plated steel sheet 2 Base steel sheet 3 Plated layer 4 Fe-Al phase 5 Fe-Al-Si phase 6 Mg-containing phase L Interface length between plated layer and base steel sheet L ₀ Surface length of base steel sheet

Claims (13)

A base steel sheet and a plating layer formed on a surface of the base steel sheet,
The plating layer comprises, in mass %,
Fe: 20.0 to 55.0%,
Mg: 0-10.0%,
Si: 0 to 10.0%,
Zn: 0-30.0%
and further comprising
Ni: 0-1.000%,
Ca: 0-4.000%,
Sb: 0 to 0.500%,
Pb: 0 to 0.500%,
Cu: 0 to 1.000%,
Sn: 0-1.000%,
Ti: 0 to 1.000%,
Cr: 0-1.000%,
Nb: 0 to 1.000%,
Zr: 0 to 1.000%,
Mn: 0 to 1.000%,
Mo: 0-1.000%,
Ag: 0-1.000%,
Li: 0 to 1.000%,
La: 0 to 0.500%,
Ce: 0-0.500%,
B: 0 to 0.500%,
Y: 0 to 0.500%,
Sr: 0-0.500%,
In: 0 to 0.500%,
Co: 0 to 0.500%,
Bi: 0-0.500%,
P: 0 to 0.500%,
W: 0 to 0.500%, and V: 0 to 0.500%
At least one of the above is contained in a total amount of 5.000% or less,
The balance has a chemical composition consisting of 20.0% or more Al and impurities,
In a cross section of the coating layer, an interface length L between the coating layer and the base steel sheet and a length L ₀ of a surface of the base steel sheet satisfy (L - L ₀ ) / L ₀ × 100 ≧ 3,
The plated steel sheet, characterized in that the plated layer contains an Fe-Al phase, and the thickness of the Fe-Al phase is 4 to 50 μm. The plated steel sheet according to claim 1, wherein (LL ₀ )/L ₀ ×100≧5. The plated steel sheet according to claim 2, wherein (LL ₀ )/L ₀ ×100≧7. The plated steel sheet according to any one of claims 1 to 3, characterized in that the Mg content in the plated layer is 0.2% or more. The chemical composition, in mass%,
The plated steel sheet according to any one of claims 1 to 4, comprising: Mg: 0.3 to 10.0%; and Si: 0 to 1.0%. The plated steel sheet according to any one of claims 1 to 5, characterized in that the thickness of the Fe-Al phase is 12 to 50 μm. The plated steel sheet according to any one of claims 1 to 6, characterized in that, in a cross section of the plated layer, a projected length T _i of an Fe-Al-Si phase in the plated layer and a length L ₀ of a surface of the base steel sheet satisfy ΣT _i /L ₀ × 100≦20. The plated steel sheet according to claim 7, wherein ΣT _i /L ₀ ×100≦1. The chemical composition contains, in mass%, Mg: 0.3 to 10.0%,
The plating layer further includes an Mg-containing phase,
The plated steel sheet according to any one of claims 1 to 8, characterized in that in a cross section of the plating layer, a surface coverage of the Mg-containing phase is 20 to 100%. The plated steel sheet according to claim 9, characterized in that the surface coverage of the Mg-containing phase is 60 to 100%. The plated steel sheet according to any one of claims 1 to 10, characterized in that the Mg content in the plated layer is 2.4% or less. The plated steel sheet according to any one of claims 1 to 11, characterized in that the Si content in the plated layer is 0.2% or more. The plated steel sheet according to any one of claims 1 to 12, characterized in that an area ratio of the MgZn ₂ phase in the plating layer is less than 10%.

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